JSFiddle - React, Tailwind, and code Playground

HTML

<div class="select-all-these">1</div>
<div class="select-all-these except-these">2</div>
<div class="select-all-these">3</div>
<div class="select-all-these">4</div>
<div class="select-all-these except-these">5</div>
<div class="select-all-these">6</div>
<div class="select-all-these">7</div>
<div class="select-all-these">8</div>
<div class="select-all-these">9</div>
<div class="select-all-these">10</div>
<div class="select-all-these except-these">11</div>
<div class="select-all-these">12</div>

JavaScript

$('.select-all-these').each(function() {
    var index = $(this).index() - $(this).prevAll('.except-these').length + 1;
    if(!$(this).hasClass('except-these') && index % 3 === 0 && index > 0) {
        $(this).text('nth-child counts all elements (1 based!)');
    }
});